Hematopoietic progenitor cells (HPCs) can home to the bone marrow (BM) after a simple intravenous injection, but the adhesive mechanisms mediating the initial interactions of human HPCs with the BM endothelium have not been evaluated in vivo. Using fluorescence intravital microscopy and homing assays in NOD/SCID mice, we show that endothelial selectins are necessary for human adult CD34(+) cell homing, since rolling on BM endothelium and retention in the BM compartment are drastically reduced (>90%) in endothelial selectin-deficient NOD/SCID mice. Comparative analyses of CD34(+) cells collected from adults and from cord blood (CB) reveal that neonatal cells display reduced rolling fractions compared with adult CD34(+) cells obtained from peripheral blood or BM, suggesting abnormal selectin ligand function on neonatal progenitors. Flow cytometric and intravital microscopy studies suggest that this defect results from nonfunctional P-selectin ligand on a subset ( approximately 30%) of neonatal CD34(+) cells. Further analyses indicate that P-selectin glycoprotein ligand-1 (PSGL-1) is expressed in a nonfunctional form among neonatal CD34(+) cells that do not bind P-selectin and that this subset is enriched in primitive CD34(+)CD38(lo/-) progenitors. These results underscore the potential to improve homing of CB CD34(+) cells to the BM by manipulation of selectins and their ligands.